A switching system is a key part of a packet switching apparatus, and is composed of a switching access unit and a switching unit. The switching access unit is composed of a source switching access unit and a destination switching access unit. The source switching access unit switches a data packet into a cell, and the destination switching access unit regroups the cell into the data packet. The switching unit takes charge of forwarding and switching the cell. Generally, there are multiple switching access units and multiple switching units in the system.
FIG. 1 is a schematic diagram illustrating connections of a three-level switching system. As shown in FIG. 1, the three-level switching system includes four service machine frames and a central machine frame. The four service machine frames are respectively service machine frame 1, service machine frame 2, service machine frame 3 and service machine frame 4. In FIG. 1, the switching unit on the service machine frame is called a service switching unit, and the switching unit on the central machine frame is called a central switching unit. The switching access unit and the service switching unit are interconnected through a high-speed serial link, so are the service switching unit and the central switching unit. The switching access unit is composed of the source switching access unit and the destination switching access unit. The source switching access unit receives a data packet from the outside and switches the data packet into a cell to send the same to the switching unit. The destination switching access unit receives the cell from the switching unit, and regroups the cell into the data packet to send the same to the outside. The service switching unit is composed of a first-level switching unit and a third-level switching unit. The first-level switching unit receives the cell from the source switching access unit and then forwards it to the central switching unit. The third-level switching unit receives the cell from the central switching unit and then forwards it to the destination switching unit. Because both the source switching access unit and the destination switching access unit are in the switching access unit, the source switching access unit and the destination switching access unit have internal buses to share information. Likewise, because both the first-level switching unit and the third-level switching unit are in the service switching unit, the first-level switching unit and the third-level switching unit have the internal buses to share information.
FIG. 2 is a schematic diagram illustrating data flows in a three-level switching system. As shown in FIG. 2, according to the direction of data flows from the source switching access unit on the service machine frame 1 to the destination switching access unit on the service machine frame 2, the cell sent by the source switching access unit on the service machine frame 1 reaches the central machine frame through the first-level switching unit on the service machine frame 1, further reaches the third-level switching unit on the service machine frame 2 through the switching unit on the central machine frame, and finally reaches each destination switching access unit on the service machine frame 2 through the switching of the third-level switching unit on the service machine frame 2. Generally, the switching unit on the central machine frame is also called a second-level switching unit, that is, there are three levels of switching units in the system.
The switching access unit and the service switching unit as well as the service switching unit and the central switching unit are interconnected through the high-speed serial links, such as the links 1 and the links 2 in FIGS. 1 and 2. These high-speed links are bidirectional links. According to the flow direction of data, the data is from the source switching access unit to the destination switching access unit.
An asymmetrical switching system means that the bandwidth in the switching unit is inconsistent for a certain destination switching access unit, which may cause long-existing local congestion in the switching unit and traffic reduction of the whole network. Packet loss may occur when the congestion is serious, so the performance of the system degrades.